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.Dd September 26, 2015
.Dt VOP_ADVISE 9
.Os
.Sh NAME
.Nm VOP_ADVISE
.Nd apply advice about use of file data
.Sh SYNOPSIS
.In sys/param.h
.In sys/vnode.h
.Ft int
.Fn VOP_ADVISE "struct vnode *vp" "off_t start" "off_t end" "int advice"
.Sh DESCRIPTION
This call applies advice for a range of a file's data.
It is used to implement the
.Xr posix_fadvise 2
system call.
.Pp
Its arguments are:
.Bl -tag -width offset
.It Fa vp
The vnode of the file.
.It Fa start
The start of the range of file data.
.It Fa end
The end of the range of file data.
A value of
.Dv OFF_MAX
indicates that the advice is to be applied up to the end of the file.
.It Fa advice
The type of operation to apply to the file data.
Possible values are:
.Bl -tag -width POSIX_FADV_WILLNEED
.It Dv POSIX_FADV_WILLNEED
Initiate an asynchronous read of the file data if it is not already resident.
.It Dv POSIX_FADV_DONTNEED
Decrease the in-memory priority of clean file data or discard clean file data.
.El
.El
.Pp
If the
.Fa start
and
.Fa end
offsets are both zero,
then the operation should be applied to the entire file.
Note that this call is advisory only and may perform the requested
operation on a subset of the requested range
.Pq including not performing it at all
and still return success.
.Sh LOCKS
The file should be unlocked on entry.
.Sh RETURN VALUES
Zero is returned if the call is successful, otherwise an appropriate
error code is returned.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EINVAL
An invalid value was given for
.Fa advice .
.El
.Sh SEE ALSO
.Xr vnode 9
